There are known an active noise control apparatus (hereinafter referred to as an “ANC apparatus”) and an active sound control apparatus (hereinafter referred to as an “ASC apparatus”) as apparatuses for controlling acoustics in relation to noise within the passenger compartment of a vehicle.
The ANC apparatus generates a canceling sound for canceling a noise such as a noise (muffled engine sound) that is generated in the passenger compartment of the vehicle by the operation (vibration) of the engine and a noise (road noise) that is generated in the passenger compartment by the contact between the wheels and the road while the vehicle is traveling, and reduces the noise with the canceling sound. Some ANC apparatuses are selectively turned on and off depending on the number of engine cylinders in operation and change frequencies to be controlled (see, for example, U.S. patent application Publication No. 2004/0258251).
An ASC apparatus generates a sound effect (quasi-engine sound) in synchronism with the muffled engine sound to enhance an acoustic effect in the passenger compartment, e.g., to emphasize a change in the speed of the vehicle (see, for example, U.S. patent application Publication No. 2006/0215846).
There has also been developed an active acoustic control system which employs an ANC apparatus and an ASC apparatus in combination (see, for example, International Publication No. WO 90/13109 and U.S. patent application Publication No. 2006/0269078). According to International Publication No. WO 90/13109, the ANC apparatus and the ASC apparatus are always in operation. According to U.S. patent application Publication No. 2006/0269078, in order to prevent the ANC apparatus and the ASC apparatus from interfering with each other, the ANC apparatus and the ASC apparatus are activated and inactivated in relation to each other depending on a combination of an engine rotational frequency [Hz] and a change per unit time in the engine rotational frequency (engine rotational frequency change) [Hz/s] (see FIG. 5 of U.S. patent application Publication No. 2006/0269078).